Elemental analysis of incubated and freshly cut mouse cerebellar slices was performed by means of energy-dispersive x-ray spectroscopy in a Hitachi H700H analytical electron microscope. Freeze-dried cryosections were probed in order to measure potassium and calcium concentrations in Purkinje cell dendritic spine organelles. The calcium content of spine cisterns from "resting" synapses was found to be 2.5 plus minus 0.5 mmol/kg dry weight, compared with a concentration of 6.7 plus minus .06 mmol/kg dry weight in fresh (i.e., stimulated) synapses. The potassium concentration in both the resting and stimulated synapses remained high and approximately constant, indicating physiological integrity of the tissue.